Device for improving the transmission properties of a bundle of electrical data lines and a system for transmitting data

ABSTRACT

The invention relates to a device for improving the transmission properties of a bundle of electrical data lines, particularly of a bundle of copper twin wires, and to a system for transmitting data that is provided with a device of this type. Said device comprises at least one adaptive filter, which generates an output signal for correcting the signal transmitted on a first data line. At least one signal tapped by a second data line or an external signal is used as a reference signal for the adaptive filter, and the corrected signal transmitted on the first data line is used as an error signal for the adaptive filter. According to the invention, the device is situated in the transmission path whereby improving the transmission properties of the cable independent from the switched-on transmission units and employed transmission techniques, particularly in the event of high bit rate transmission.

This application claims the benefit of the earlier filed InternationalApplication No. PCT/EP02/07783/, International Filing Date, Jul. 15,2002, which designated the United States of America, and whichinternational application was published under PCT Article 21(2) as WOPublication No. WO 03/009490 A3.

The invention relates to an apparatus for improvement of thetransmission characteristics of a bundle of electrical data lines asclaimed in claim 1, and to an arrangement for data transmission asclaimed in claim 11.

When data is being transmitted via two or more data lines which are inimmediate physical proximity of one another, the problem of crosstalkarises, in particular near-end and/or far-end crosstalk (NEXT and FEXT,respectively). Problems such as these occur in particular in thecircuitry of the copper cables (which are actually designed forlow-frequency transmission) in the access area from a telecommunicationsnetwork to high bit-rate transmission systems. High bit-ratetransmission systems such as xDSL (Digital Subscriber Line, for exampleHDSL, SDSL, ADSL, VDSL) have been developed in order to make it possibleto use the existing infrastructure of double copper wires in localnetwork cables and other telecommunication cables for the transmissionof high-speed data streams. The multiple or mixed connection of thesecables to different transmission techniques frequently leads to theproblem of spectral incompatibility. In this case, two or moretransmission devices which are connected to a common cable interferewith one another. The telecommunications cables can frequently not beused optimally. The transmission capacity which can be achieved for apredetermined transmission quality is limited. In consequence, it iseither not possible to connect all of the wire pairs, or there arerestrictions on the distances that can be covered.

The influence of the interference can be kept low by matching thetransmission appliances that are connected to the cable bundle to oneanother, in particular by synchronizing them. However, this is complexand leads to restrictions relating to the modem and transmissiontechniques that are used.

By way of example, a compensation method for a pool comprising Nidentical logical MDSL modems is known from U.S. Pat. No. 5,970,088.This is used to compensate for the NEXT interference. The modemtransmitters are synchronized to a clock generator. The signals whichare received by the N modems are suppressed, after reception, by meansof a compensation circuit. The compensation circuit comprises Nsubunits, each of which has its own associated modem. One subunitcomprises N adaptive filters, whose output signal is used for correctionof the signal which has already been received and digitized by themodem. The transmission signal which is transmitted by in each case oneof the N modems is used as a reference signal for the adaptive filters.The output signals from all the adaptive filters in a subunit arecombined, and are used for correction of the received signal which hasalready been received. The compensation method which is described inU.S. Pat. No. 5,970,088 requires N identical modems, and is thereforenot suitable for suppression of undefined systems. The transmissioncharacteristics of the cable itself remain unchanged, and the signalwhich has already been received is subsequently processed.

The invention is based on the object of specifying an apparatus by meansof which the transmission characteristics of a bundle of electrical datalines are improved, so that this makes it possible to providesuppression between the lines, and thus to increase the transmissioncapacity, irrespective of the appliances which are connected to the datalines.

The object is achieved by an apparatus for improvement of thetransmission characteristics of a bundle of electrical data lines havingthe features of claim 1. The object is also achieved by an arrangementfor data transmission by means of an apparatus such as this, as claimedin claim 11. Advantageous developments of the invention are specified inthe dependent claims, in the description and in the drawings.

According to the invention, the apparatus comprises an electroniccircuit which is arranged between two connections. The electroniccircuit comprises at least one adaptive filter, by means of which asignal which is transmitted on a first line is suppressed. Theconnections are used for connection to one or more cable bundles and/orto one or more transmission appliances, which may be terminals. At leastone of the connections, for example the input to the apparatus, ispreferably connected to a cable bundle such that the data lines in thecable bundle make contact with the electronic circuit, or with its datalines. The other connection may be designed for connection of furthercable bundles, or for connection of transmission appliances.

When in use, this simulates the output of the cable bundle which isconnected to the input of the apparatus. As a result of the connections,the apparatus acts as a “black box”, which can be arranged at suitablepoints within the transmission path, for example between the cable endsand their transmission appliance, or between two cables. For thispurpose, the apparatus preferably also has a housing, in which thecircuit is arranged. The apparatus can be installed universally and withlittle effort.

Since the apparatus is arranged within the transmission path, that is tosay at one or both ends of the cable bundle, the transmissioncharacteristics of the cable bundle itself are improved. There is noneed for any knowledge about the transmission techniques or about theconnected transmission appliances that are used. The apparatus iscompletely transparent and is independent of the structure of theconnected appliances. In contrast to known suppression circuits whichare arranged within the transmission appliances, no interventionwhatsoever is required in the transmission appliances according to theinvention. The process is carried out using the signals which thetransmission appliance emits or receives at its conventional interfacesto the cable bundle. Any desired transmission appliances, includingfuture transmission appliances, can thus be used. The network operatorsand customers need therefore not be subjected to any restrictions withregard to the appliances and transmission techniques which are used, inorder to achieve a high transmission performance and quality.

Adaptive filters for reduction of and compensation for interference ofan unknown nature on a signal line are known per se. These aredescribed, for example, in B. Widrow and S. Stearns, “Adaptive SignalProcessing”, Prentice-Hall, Inc., New Jersey, 1985. A useful signalwhich is to be suppressed and is transmitted on a signal line iscomposed of the signal without interference and of the interferenceinfluence from the unknown signal source. A signal which is correlatedwith this signal source is supplied as a reference signal to an adaptivefilter. The filter produces a compensation signal, which is subtractedfrom the useful signal. The compensated useful signal obtained in thisway is supplied as a fault signal to the filter. The filter parametersare set via a suitable adaptation algorithm such that the power in thecompensated useful signal is minimized and which thus ideallycorresponds to the signal without interference.

The invention can be implemented in a large number of embodiments, someof which are illustrated in the drawings. Each line to be suppressed hasat least one associated adaptive filter that produces a compensationsignal which is subtracted from the useful signal transmitted on theline. At least one signal which is tapped off from a further line isused as a reference signal for the adaptive filter, so that theinterference influence, for example NEXT and/or FEXT, on the signalswhich are transmitted on this line is reduced. In this variant of theinvention, one adaptive filter is provided for each pair of lines to bedecoupled. It is also possible to tap off the reference signals fromonly those lines that are used for transmission.

In order to reduce the number of adaptive filters, the signals which aretapped off from the lines that are subject to interference are, in afurther variant, combined to form a common reference signal, with, forexample, their direct or weighted sum being formed. This referencesignal is then supplied to a common adaptive filter.

In a further advantageous embodiment, the reference signal is tapped offfrom a line which is not connected. This signal is a map of all theinterference influences on adjacent lines that are used for transmissionor reception. The signal which is tapped off thus essentially alsoindicates the corresponding interference influences on the line to besuppressed. If it is used according to the invention as a referencesignal for an adaptive filter, this allows rapid adaptation of thefilter, and hence rapid compensation.

In a further preferred embodiment, a reference signal is tapped offoutside the cable bundle, for example from an external antenna. Thisalso allows external interference influences on individual data lines oron all the data lines to be compensated for.

In order to install the apparatus according to the invention in thetransmission path, there are preferably two connections, with oneconnection being used, for example, for connection of the cable bundleto be suppressed and the other being used for connection of a cablebundle which leads to one or more subscribers, or for direct connectionof transmission appliances, for example xDSL appliances. The apparatusalso has a bundle, which connects the connections, of apparatus-internalconnecting lines, with these connecting lines having at least oneconnecting point via which reference and/or fault signals can be tappedoff and/or correction signals can be supplied. There is therefore noneed to carry out any modifications on the external cable itself.

In order to suppress wire pairs, the apparatus furthermore has hybridcircuits, by means of which the transmission and received signals whichare jointly located on each connected wire pair in the cable bundle areseparated, and are each associated with their own internal connectingline. The reference and fault signals for setting the adaptive filterscan then be tapped off at a suitable point on these connecting lines,and the correction signals that are produced can be supplied. The dataflow is not interrupted in the event of a failure on the adaptivefilters, for example in the event of a power failure.

In order to achieve compensation that is as good as possible, each dataline is decoupled from every other data line by, for example, providingan adaptive filter for every other data line or by supplying the signalsthat are tapped off from the further data lines as a reference signal toa common adaptive filter. Since the crosstalk is highly dependent ondistance, it may for many applications be sufficient to providesuppression only between the immediately adjacent data lines. In thecase of telecommunications cables, two pairs of double copper wires arefrequently twisted with one another. Two or more such star groups offour are contained in one common basic bundle. Two or more such basicbundles in turn form the telecommunications cable. In a case such asthis, at least the data lines in one star group of four and,particularly preferably those in a basic bundle, are preferablysuppressed with respect to one another.

A large number of adaptation methods for setting the adaptive filtersare known, which all have the aim of minimizing the compensated signal(fault signal). For example, it is possible to use a least mean square(LMS) algorithm, the RLS algorithm (Recursive Least Squares Algorithm),self-recovering equalization or blind equalization algorithms, or anyother suitable method. The power minimization method has the advantagethat the apparatus according to the invention is transparent withrespect to the transmission rates, line codes and other transmissionparameters, and that no training or synchronization processes arerequired.

Exemplary embodiments of the invention will be described in thefollowing text and are illustrated in the drawings, in which,illustrated purely schematically:

FIG. 1 shows the basic arrangement of two apparatuses according to theinvention within a transmission path;

FIG. 2 shows a first example of a circuit arrangement according to theinvention with a reference point for each wire pair;

FIG. 3 shows a further example of a circuit arrangement according to theinvention with two reference points for each wire pair;

FIG. 4 shows a further example of a circuit arrangement according to theinvention with in each case one addition element for transmission andreceived signals on adjacent lines;

FIG. 5 shows a further circuit arrangement according to the inventionhaving an addition element for all interference signals.

FIG. 1 shows an arrangement according to the invention for datatransmission in an application with a central switching device 300 andtwo or more transmission appliances M₁, M₂ . . . M_(n) and M_(1′), . . .M_(2′), . . . M_(n′)which are preferably xDSL modems. The transmissionappliances M₁ and M_(1′), M₂ and M_(2′) . . . M_(n) and M_(n′) representany given 1 . . . n xDSL connections, which are connected via in eachcase one wire pair S₁/E₁, S₂/E₂ . . . S_(n)/E_(n). The reference symbolsS_(iT), S_(ik), S′_(iT), S′_(iK), E_(iT), E_(ik), E′_(iT), E′_(ik) eachdenote the subscriber-end or cable-end element of the transmission orreception path at the two cable ends. The wire pairs are combined toform a bundle 200 of lines, and are located in the immediate physicalvicinity of one another in the area of this cable. In order tocompensate for the interference influences between them, one apparatusor if required two apparatuses according to the invention 100 and 100′is or are located in the transmission path between the modems and thewire pairs of the cable. At their switching end, the individual wirepairs in the cable bundle 200 are connected to the transmissionappliances M_(1′), M_(2′) . . . M_(n′). At their subscriber ends, theindividual wire pairs are connected to the transmission appliances M₁,M₂ . . . M_(n). A first connection 400, 400′ is respectively used forthis purpose. A second connection 500, 500′ is respectively used forconnection of the cable bundle. The subscriber-end wire pairs may alsobe combined to form a further cable bundle.

FIG. 2 shows a first example of a circuit arrangement which can be usedin an apparatus according to the invention. Two or more wire pairsS₁/E₁, S₂/E₂ . . . S_(n)/E_(n) which are connected both to thetransmission signal and to the received signal of the associated modempair M₁-M_(1′), M₂-M_(2′) . . . M_(n)-M_(n′) are suppressed with respectto one another and with respect to an external interference source. Forthis purpose, each wire pair in the cable bundle which contributes tointerference with the other wire pairs is split by means of highlyeffective hybrid circuits G_(K1), G_(K2) . . . G_(Kn) at the cable endand G_(T1), G_(T2) . . . G_(Tn) at the subscriber end into thetransmission lines S₁, S₂ . . . S_(n) and the reception lines E₁, E₂ . .. E_(n). Each reception line E₁, E₂ . . . E_(n) has n adaptive filters Aassociated with it for suppression of the received signals, where nindicates the number of wire pairs that are present. The adaptivefilters receive, as their reference signal, the signal which is tappedoff at the cable-end junction point 1, 2 . . . n of in each case oneadjacent wire pair. This signal comprises the signal which istransmitted and received on this wire pair, as well as the interferencesignals from the adjacent wire pairs. Two wire pairs are thus in eachcase associated with one adaptive filter in order to suppress the signalwhich is transmitted on the first reception line, for example E₁, withrespect to the signal which is transmitted on a further wire pair, forexample S₂/E₂. A further adaptive filter A_(x) is provided in order tocompensate for any external interference influence. Overall, thiscircuit arrangement according to the invention has a matrix comprisingn×n adaptive filters, which are used to compensate for all of theinterference influences interactively. Since the reference signal istapped off at the junction point between adjacent wires, compensation isprovided both for the near-end crosstalk and for the far-end crosstalk.

FIG. 3 shows a further circuit arrangement in which, for example, tworeference points 2 a, 2 b; . . . na, nb and the corresponding number ofadaptive filters are used for suppression of the received signal on thewire pair S₁/E₁ for each adjacent wire pair that is subject tointerference. The transmission and received signals are used separatelyas reference signals. Furthermore, an adaptive filter is provided forthat specific wire pair, which taps off as the reference signal thesignal which is transmitted from its own modem at the reference point 1a, and thus provides echo compensation. As in the case of the circuitarrangement shown in FIG. 2, a further adaptive filter is provided inorder to compensate for external interference.

FIG. 4 shows a further example of a circuit arrangement according to theinvention, with only one wire pair and the associated adaptive filtersbeing illustrated. Each wire pair generally has three associatedadaptive filters A_(s), A_(E), A_(x). One filter A_(s) receives as thereference signal the sum (added up in an addition element Σ_(s)) of allthe transmission signals from all of the wire pairs, or only from theother wire pairs. The second filter A_(E) receives as a reference signalthe sum (added up in an addition element Σ_(E)) of all the receivedsignals on the other wire pairs. A filter A_(x) is used to compensatefor external interference. The reference points 1 a, 1 b. . . na, nbcorrespond to those in FIG. 3. This arrangement can be implementedeasily and cost-effectively, since the number of adaptive filters isless than that in the examples shown in FIGS. 2 and 3.

FIG. 5 illustrates a further simplification, in which only one adaptivefilter A_(Σ) is provided for all of the interference signals which aretapped off from adjacent wire pairs. These interference signals areadded up in an addition element Σ, with the sum signal being used as areference signal. A filter A_(x) is once again used to compensate forexternal interference.

In a further simplification of the circuit arrangement, it is feasiblefor the function of A_(x) to be integrated in A_(Σ), so that A_(x) maybe omitted.

A further simplification is obtained by the supplier using anunconnected wire pair for the reference signals. If this wire pair is ina suitable position within the cable, all of the interference signalswill, of course, be present on it. There is therefore no need for theaddition element, which can be replaced by an operational amplifier towhich the reference wire pair is connected and which provides theinterference signal in a decoupled form to each transversal filter. Afilter A_(x) is once again used to compensate for external interference.

In a further possible embodiment of the predetermined solution variant,the reference wire pair and the external interference signal areconnected to an addition element which now has only two inputs. Inconsequence, this saves the adaptive filter A_(x).

1. An apparatus for improvement of the transmission characteristics of abundle of electrical data lines, in particular of a bundle of copperdouble wires, having an electronic circuit arranged between a firstconnection being used for connection to an external bundle of electricaldata lines and a second connection being used for connection to datalines of a further external bundle of electrical data lines and/or toone or more external transmission appliances, the electronic circuitcomprising at least one adaptive filter producing an output signal forcorrection of a received signal of a first data line of the externalbundle of electrical data lines, a reference signal for said at leastone adaptive filter being at least a signal which is tapped off from asecond data line of the external bundle of electrical data lines and/oran external signal, and a fault signal for said at least one adaptivefilter being the corrected received signal of the first data line of theexternal bundle of electrical data lines; wherein a bundle connects thefirst and second connections, of apparatus-internal connecting lines,with the apparatus-internal connecting lines having at least oneconnecting point, via which reference and/or fault signals can be tappedoff and/or correction signals can be supplied.
 2. The apparatus asclaimed in claim 1, wherein each data line to be suppressed has two ormore associated adaptive filters, which each receive a reference signal,which is tapped off from in each case a further data line of theexternal bundle of electrical data lines, as reference signals.
 3. Theapparatus as claimed in claim 1, wherein said at least one adaptivefilter receives a reference signal which is a combination of the signalwhich is tapped off from the second data line of the external bundle ofelectrical data lines and one or more signals which are tapped off fromfurther data lines of the external bundle of electrical data lines,preferably the sum of these signals.
 4. The apparatus as claimed inclaim 1, wherein the second data line is not used for data transmission.5. The apparatus as claimed in claim 1, wherein the external signal istapped off from an antenna which is arranged outside the external bundleof electrical data lines.
 6. The apparatus as claimed in claim 1,wherein the fault signal is applied to a first connecting line which isassociated with the first data line, and the reference signal is tappedoff from a second connecting line, which is associated with the seconddata line, for suppression of the first data line.
 7. The apparatus asclaimed in claim 1, wherein the bundle of electrical data linescomprises wire pairs which are each formed from two electrical datalines and on which transmission and received signals can be transmittedjointly, with the apparatus for each wire pair having twoapparatus-internal connecting lines and corresponding hybrid circuits,by means of which the transmission and received signals are separatedand are in each case associated with their own apparatus-internalconnecting line.
 8. The apparatus as claimed in claim 6, wherein thereceived signal is in each case suppressed by using as the referencesignal the transmission signal or the combined transmission and receivedsignal on at least one further wire pair, or a combined transmission ortransmission and received signal on two or more further wire pairs, bytapping off from the appropriate apparatus-internal connecting lines. 9.The apparatus as claimed in claim 6, wherein the wire pairs are used fordata transmission based on an xDSL method.
 10. A bundle of electricaldata lines (cables), transmission appliances and at least one apparatusas claimed in claim 1, which is arranged in at least one of the cableends in the transmission path.
 11. The arrangement as claimed in claim10 wherein the apparatus is an electrical circuit device formed on amain distribution board.
 12. The arrangement as claimed in claim 10,wherein xDSL devices are connected to the cable and/or directly to theapparatus.
 13. The apparatus as claimed in claim 2, wherein one of thefurther data lines of the external bundle of electrical data lines isnot used for data transmission.
 14. The apparatus as claimed in claim 2,wherein a further associated adaptive filter receives as referencesignal an external signal tapped off from an antenna which is arrangedoutside the external bundle of electrical data lines.
 15. The apparatusas claimed in claim 3, wherein the combination additionally comprises anexternal signal tapped off from an antenna which is arranged outside theexternal bundle of electrical data lines.